FIG. 1 shows a diagrammatic representation of a rear projection set-up known from the prior art by means of a projector P, where a Fresnel lens F is disposed in front of the projection screen P. A brightness which is more regular for a viewer V from a particular angle of view can be brought about in this way.
The projection screen P consists of a plastic material, for example polymethyl methacrylate (PMMA), associated with a diffuser. The diffuser is obtained e.g. by sand blasting the output surface (the surface seen by the viewer) of the projection screen or by mixing small particles (e.g. titanium dioxide) in a resin before polymerization.
In tiled rear projection setup, several rear projection modules each with a screen P are disposed next to each other is a checkerboard manner to form a larger projection screen as seen on FIG. 2.
FIG. 2 also shows schematically how the picture elements are arranged on the projection screen P. The projection of an image depicted by a light valve (e.g. LCOS, LC or micro-mirrors aka DMD) unit is involved. Three sub-pixels green, red and blue lie close to one another and produce in their totality a picture element of the image displayed. The picture elements follow one another with a spacing A. The broken line indicates the inner area of the projection screen P onto which light can be projected in projection set-up known from the prior art. It becomes clear from this representation that it is not necessary to construct the rear projection modules in such a way that the picture elements butt directly against one another at the edges of the projection screens P, since it is sufficient by virtue of the pixel structure of the displayed image if the realizable spacing between two adjacent picture elements projected onto adjacent projection screens corresponds roughly to the spacing A of the picture elements on the projection screen P. In the prior art the spacing between two adjacent picture elements projected onto adjacent projection screens is largely determined by the distance separating two adjacent projection screens P. The space between two adjacent projection screens P is often referred to as the seam. When the light valve is a DMD, there are not necessarily red, green and blue sub-pixels. In that case, each of the color components is projected sequentially and occupies the entire area of a pixel. Instead of being projected sequentially, the color components can be projected simultaneously on top of each other, each color component of an image is formed by a DMD, the projector having 3 DMDs to form a color image.
The minimum distance between two adjacent projection screens depends on the clearance needed to allow thermal and humidity expansion of the projection screens P without misalignment, bowing or damaging of the projection screens P.
The problem of thermal and humidity expansion and the relative movement of tiles with respect to each other and its impact on the inter-tile seams is well known in the art too.
In U.S. Pat. No. 5,400,178 “Screen unit for rear projection picture display system, method for producing the same and component to be used for assembling the screen unit” the problem of thermal expansion is addressed by using materials with approximately the same coefficients of thermal expansion. Nevertheless, the differential thermal expansion is still too large for seamless tiled displays where the width of the inter-tile seams must be equal or smaller than the distance between two adjacent pixels on the same tile.
In U.S. Pat. No. 5,185,677 “Transmission type projection screen assembly” a tensioning mechanism is used. The mechanism comprises force receiving blocks made of a material with a coefficient of thermal expansion substantially equal to that of the sheets of which the screen tiles are made.
But for tiled display with more than one row of adjacent screens, the tensioning mechanism cannot be hidden from the viewer. The mechanism proposed in U.S. Pat. No. 5,185,677 thus does not allow the assembly of large, multi-rows seamless tiled displays.
The effect of atmospheric conditions is important not only for multi-screen displays assembled from basic units but also for the multi-screen displays from the prior art and in which larger screens are obtained by joining together smaller screens, for example by adhesive bonding.
The basic units of the prior art which have been described above often have a frame around the screen, which frame holds this screen flat, often also serves to attach the screen to the housing of the basic unit and protects the edges of the screen from damage. This frame prevents an image from being projected all the way as far as the outer edge of the front side of the basic unit and makes the visual joint or seam between the basic units larger. EP-0 650 295 and EP-0 523 427 describe basic units of this nature and the assembly of a plurality of basic units of this nature to form a multi-screen display. A solution for allowing images to be projected right up to the edge of the front side of the basic unit is described in WO 95/28664 and consists in a special treatment of the outer edges of the screen and a special attachment of the said screen to a supporting structure, so that the light from the projector is able to reach the front edges of the screen—which are also the front edges of the basic unit—without being impeded. In principle, it is possible to achieve a perfect connection between screens of basic units at a specific temperature. However, the edges of a basic unit of this nature are fragile during transportation and the other drawbacks which are inherent to the assembly of basic units continue to exist.
Another solution is to adhesively bond together relatively small optical screens to form larger screens. Adhesively-bonded larger screens of this nature cannot be produced at the location where the projection screen is to be installed, but rather can be produced only at the premises of the constructor, following which they have to be transported in very robust and large packaging. In order to prevent damage and contamination to the screens during attachment to a supporting structure, installation has to take place with the greatest possible care. Under the influence of atmospheric conditions (ultraviolet radiation, repeated temperature changes, absorption of humidity, oxidation . . . ) the mechanical and optical properties of the glue will change and the seams will become more discernible.
When gluing to produce an adhesive bond, a pressure is used to squeeze the glue out into a thin continuous film between layers, to force air from the joint, to bring the surfaces into intimate contact with the glue, and to hold them in this position during the setting of the glue.
This is the traditional gluing strategy because
a) The glue material itself is not very strong so it is not a good idea to have a large thickness of glue. Instead the glue should be a very thin layer with the glue sticking very well to a well prepared surfaces.
b) To do this the surfaces must be very flat, perfectly clean and must be clamped together as strongly as possible until the glue dries
The glue is squeezed out (step 1) and also the surfaces have to be held together under pressure (step 2).
Step 1 can be messy during production and requires the surfaces to be cleaned and step 2 slows down production and hence increases production time and increases costs of production.
The pressure applied can break fragile substrates, e.g. made from glass. In order to reduce the weight of large tiled displays there is an interest in reducing the thickness and weight of all components.
Liquid glue is not compatible with the slanted/lateral gluing surfaces. The glue would have an irregular thickness (which would impact the gap/seam between display tiles) and on occasion is known to flow/to seep between layers (e.g. between a Fresnel lens and a substrate above it). Such glue can forms stains and visible artefacts.
One advantage of bounded heat activated adhesive such as heat activated adhesive tapes is that the heat activity (i.e. a combination of temperature and activation time) can be well controlled such that thermal damage to other components can be reduced or avoided while still having excellent structural adhesive properties. For example, bounded heat activated adhesive such as heat activated adhesive tapes as used with embodiments of the present invention can be activated at temperatures of up to 120° C. with an activation time of less than 90 sec. A higher activation time can be used if the temperature is lower. The activation time becomes less of a problem if the temperature is reduced to typical polymer glass temperature such as 70° C.-80° C. Such very low temp systems are less preferred. So that a reasonable range of activation temperature is 100 to 140° C. with an activation time of less than 30 seconds at 140° C. and less than 90 seconds at 120° C. activation time.
The screen panels are preferably optical panels, such as a Fresnel lens, a lenticular or a combination of the two and have a perfectly rectilinear edge. They may comprise a plurality of layers, of which at least one layer is attached to the attachment plates with the aid of joining wires. In order to draw the screen panels towards one another and to position them with respect to one another, and in order to fix these screen panels to and position them with respect to the attachment plates, use is preferably made of rigid joining wires which are U-shaped and are made from metal or plastic.
The seams between the screen panels which have been drawn towards one another are minimal, with a size of less than half a millimeter. Drawing the panels together by means of joining wires ensures that the seams between the screens are always, and remain, minimal, in spite of manufacturing tolerances and under changing climatological conditions, such as temperature and relative atmospheric humidity. Any differences in expansion/shrink between the screen panels, on the one hand, and the supporting structure of the projection screen, on the other hand, are absorbed by the fact that the attachment plates which join together the screen and the supporting structure are deformable and/or have a movable join to the supporting structure, and the fact that the joining wires can be deformed to a limited extent in order to keep the join between the screen panels optimal. As a result, the screen can move with respect to the supporting structure, within certain limits, in such a way that the seams remain minimal. However, the stitching wires in the stitched screens disclosed in EP 1 012 666 B1 “PROJECTION SCREEN FOR IMAGE REPRODUCTION DEVICES WHICH ARE POSITIONED NEXT TO AND/OR ABOVE ONE ANOTHER” are easily discernible. Also the installation effort is high, projectors must be readjusted if environmental conditions are changing and the wall size is practically limited